Storing energy in the form of compressed gas has a long history and components tend to be well tested and reliable and to have long lifetimes. The general principles for compressed gas energy storage are that generated energy (e.g., electric energy) is used to compress gas and thus convert the original energy to pressure potential energy; the energy is later recovered in a useful form (e.g., converted back to electric energy) via appropriate gas expansion. Advantages of compressed gas energy storage include low specific-energy costs, long lifetime, low maintenance, reasonable energy density, and good reliability.
As the world's demand for electric energy increases, the existing power grid is being taxed beyond its ability to meet continuous demand. In certain parts of the United States, inability to supply peak demand has led to inadvertent brown-outs and blackouts due to system overload and to deliberate “rolling blackouts” of non-essential customers to shunt the excess demand. For the most part, peak demand occurs during the daytime hours (and during certain seasons, such as summer), when business and industry employ large quantities of power for running equipment, heating, air conditioning, lighting, etc. At night, demand for electricity is often reduced significantly, and the existing power grid in most areas can usually handle this load without problem.
To prevent power shortages at peak demand, users are asked to conserve where possible. Power companies often employ rapidly deployable gas turbines to supplement production to meet demand. However, these units burn expensive fuels, such as natural gas, and have high generation costs when compared with coal-fired systems and other large-scale generators. Thus, they are only a partial solution in any growing region and economy. The ultimate solution involves construction of new power plants, which is expensive and has environmental side effects.
Also, because most power plants operate most efficiently when generating a relatively continuous output, the difference between peak and off-peak demand often leads to wasteful practices during off-peak, such as over-lighting of outdoor areas, as power is sold at a lower rate during off peak.
In each case, the balancing of power production or provision of further capacity rapidly on-demand can be satisfied by a local backup generator. However, such generators are often costly, use expensive fuels such as natural gas or diesel, are noisy, and are environmentally damaging due to their inherent emissions.
Various techniques are available to store excess power for later delivery. One technique involves the use of driven flywheels that are spun up by a motor drawing excess power. When the power is needed, the flywheels' inertia is tapped by the motor or another coupled generator to deliver power back to the grid and/or customer. The flywheel units are expensive to manufacture and install and require costly maintenance on a regular basis.
Another approach to power storage is batteries. However, most large-scale batteries use a lead electrode and acid electrolyte. These components are environmentally hazardous. Many batteries must be arrayed to store substantial power, and the batteries have a relatively short life (3-7 years is typical). Thus, to maintain a battery storage system, a large number of heavy, hazardous battery units must be replaced on a regular basis and the old batteries must be recycled or properly disposed of.
Energy can also be stored in ultracapacitors. A capacitor is charged by line current so that it stores a potential, and this potential can be discharged rapidly when needed. Appropriate power-conditioning circuits are used to convert the power into the appropriate phase and frequency of AC. However, a large array of such capacitors is needed to store substantial electric power. Ultracapacitors, while more environmentally friendly and longer lived than batteries, are substantially more expensive and still require periodic replacement due to the breakdown of internal dielectrics, etc.
Another approach to storage of energy for later distribution involves the use of a large reservoir of compressed air. By way of background, a Compressed-Air Energy Storage (CAES) system is shown and described in the published thesis entitled Investigation and Optimization of Hybrid Electricity Storage Systems Based Upon Air and Supercapacitors, by Sylvain Lemofouet-Gatsi, Ecole Polytechnique Federale de Lausanne (20 Oct. 2006), Section 2.2.1, the disclosure of which is hereby incorporated by reference in its entirety. As stated by Lemofouet-Gatsi, “the principle of CAES derives from splitting of the normal gas turbine cycle—where roughly 66% of the produced power is used to compress air-into two separated phases: The compression phase where lower-cost energy from off-peak base-load facilities is used to compress air into underground salt caverns and the generation phase where the pre-compressed air from the storage cavern is preheated through a heat recuperator, then mixed with oil or gas and burned to feed a multistage expander turbine to produce electricity during peak demand. This functional separation of the compression cycle from the combustion cycle allows a CAES plant to generate three times more energy with the same quantity of fuel compared to simple cycle natural gas power plant.
CAES has the advantages that it doesn't involve huge, costly installations and can be used to store energy for a long time (more than one year). It also has a fast start-up time (9 to 12 minutes), which makes it suitable for grid operation, and the emissions of greenhouse gases are lower than that of a normal gas power plant, due to the reduced fuel consumption. One of the main drawbacks of CAES is the geological structure reliance, which substantially limits the usability of this storage method. In addition, CAES power plants are not emission-free, as the pre-compressed air is heated up with a fossil fuel burner before expansion. Moreover, CAES plants are limited with respect to their effectiveness because of the loss of the compression heat through the inter-coolers, which must be compensated during expansion by fuel burning. The fact that conventional CAES still rely on fossil fuel consumption makes it difficult to evaluate its energy round-trip efficiency and to compare it to conventional fuel-free storage technologies . . . .”